Microelectromechanical systems (MEMS) are the technology of forming micro-structures with dimensions in the micrometer scale (one millionth of a meter). Significant parts of the technology have been adopted from integrated circuit (IC) technology. Most of the devices are built on silicon wafers and realized in thin films of materials. There are three basic building blocks in MEMS technology, which are the ability to deposit thin films of material on a substrate, to apply a patterned mask on top of the films by photolithographic imaging, and to etch the films selectively to the mask. A MEMS process is usually a structured sequence of these operations to form actual devices.
MEMS applications include inertial sensors applications, such as accelerometers for measuring linear acceleration and gyroscopes for measuring angular velocity. Other MEMS applications include optical applications such as movable mirrors, and RF applications such as RF switches and resonators.
For MEMS systems, usually, a floating mechanical structure is formed to provide the pre-defined working function. Below the floating structure, a gap or cavity is formed thereat, and at least a beam or spring is formed to link the floating structure to an anchor area that is fixed to the carrier substrate. For fabrication, the formation of the gap or cavity can be developed by various methods, especially for inertial sensors applications. The popular methods include Silicon on Insulator (SOI) MEMS with oxide sacrificial layer, polysilicon with oxide sacrificial layer, and Si bonding on cavities.
A typical MEMS system using a SOI (silicon on insulator) wafer as the manufacturing material may comprise a silicon carrier wafer including an oxide layer formed thereon. A device silicon wafer may be bonded to the oxide layer. A plurality of etch windows may be formed by removing a part of the device silicon wafer, which may be achieved by dry or wet etch process for Si. This etch step is to define the MEMS structure area and anchor area and additionally a supporting beam or spring Through the etch windows, the last etch step is adopted for removing the oxide layer underneath the MEMS structure to form the gap or cavity. The etch profile is isotropic and therefore, some oxide layer under the anchor area is also etched to form an undercut, which makes the anchor geometry design more difficult to control.
Further, the SOI wafer approach is much more expensive than the common used silicon wafer and this SOI scheme can't afford the interconnect layers underneath the MEMS structure for sensing electrodes or electrical routing. Therefore there is a need for exploring alternative designs.